As I suspected in my first post on Fischer - Stein, Sousse 1967, the 11th and 13th World Champions were in such close agreement regarding the turning points of the game that there is little to weigh. The biggest point of disagreement was in the opening, where Kasparov had >35 more years of theory to draw on than Fischer had.
Both players were recognized experts in the Ruy Lopez during their respective peak playing years, and I learned a lot in studying Kasparov's notes. Retaining Kasparov 's explanations, but leaving out his detailed variations, gives a good overview of some key ideas in the Closed Lopez, and the following indented comments are all his. The game started 1.e4 e5, where Kasparov wrote,
Stein's avoidance of his usual Sicilian Defense, of his favorite Accelerated Dragon, is a first moral concession to the opponent.
That thought takes on special significance later in the game; see my last quote from Kasparov. There followed 2.Nf3 Nc6 3.Bb5 a6 4.Ba4 Nf6 5.O-O Be7 6.Re1 b5 7.Bb3 d6 8.c3 O-O 9.h3, reaching the position shown in the first diagram, one of the best known positions in chess. Stein continued 9...Bb7, and Kasparov wrote,
Rare for that time. Earlier Stein had employed 9...Nd7, but 9...Na5, 9...h6, or 9...Nb8 was the main continuation.
Curious about current theory, I made a quick survey of the moves played by top players nowadays. I discovered that 9...Nb8 appeared in about 50% of recent games and 9...Na5 in 25%; the moves 9...Bb7 & 9...Re8 make up the rest with other moves played only rarely. After Stein's 9...Bb7, there followed 10.d4 Na5.
The time of the Zaitsev Variation, 10...Re8, had not yet arrived.
These days, 10...Re8 is just about the only move the top players consider. After 10...Na5, the game continued 11.Bc2 Nc4.
A very intricate plan: Black wants to take his opponent away from the well-trodden paths. The Knight is headed for d7.
12.b3 Nb6 13.Nbd2 Nbd7.
An inferior version of the Breyer Variation [9...Nb8] has arisen: White has already advanced his b-Pawn, but Black has not yet played ...Re8. [...] 13...Re8 was more logical. [...] Stein's play between the 9th and 13th moves is typical of the level of opening theory at that time, when there existed a rather light-hearted approach to the choice of variation for a forthcoming game: the main thing was to avoid routine, and then let's see what happens!
The line in the Breyer that Kasparov refers to is 9...Nb8 10.d4 Nbd7 11.Nbd2 Bb7 12.Bc2 Re8. Black is two tempi ahead of the current game, where there followed 14.b4; Fischer gave this move a '!', Kasparov a '!?'.
White's move is also typical of the level of opening theory at that time. Subsequent experience with the Breyer and Zaitsev Variations showed that the inclusion of b2-b3 is useful with 14.d5 c6 15.c4, or first 14.Nf1 Re8 15.Ng3. The fashion for handling 'Spanish' positions was set by Karpov, and after him, now without any doubts, there followed Anand, Leko, and others.
Now we find both players making concessions to pursue their respective goals; 14...exd4.
In search of counterplay Stein is obliged to concede the center.
15.cxd4 a5 16.bxa5.
Conceding the Queenside, in order to attack on the Kingside.
Black played 16...c5, arriving at the second diagram.
On 17.e5, Fischer once again gave his move a '!' against Kasparov's '!?'.
The thirst for an open fight! But perhaps the cool-headed 17.Bb2, followed by a2-a4 would have been more unpleasant for Black, gaining the c4-square for the Knight.
(Quoting Kmoch:) The text is more active than 17...Ne8, but also more dangerous for Black's King.
18.dxe5 Nd5 19.Ne4.
Again aggression. If 19.a4 or 19.Bb2, then 19...Rxa5 is quite acceptable.
19...Nb4 20.Bb1 Rxa5 21.Qe2.
The critical moment of the entire game has been reached.
(Quoting Fischer:) Quite possibly "the losing move". It is better to reserve this Knight for the defense of the Kingside. More prudent is 21...Re8 with ...Nf8 in the offing.
(Kasparov:) And indeed, after this Black would have retained a solid position. [...] It seems to me that Stein's fatal decision was a consequence of his 'Sicilian' habit, where Black often saves himself from attack on his King by the rapid creation of counterplay on the Queenside, and his insufficient experience in the 'Spanish', where on the contrary, Black must in the first instance take care of his King. as a result, Stein's sense of danger did not operate.
This game made a big impression on me when I first saw it many years ago. The Knight on b6 soon goes to c4, joining his fellow Knight on b4. The two Black Knights then sit on the wrong side of the board while their King undergoes a ferocious attack. If I had had access to Kasparov's notes the first time I saw the game, I certainly would have understood it more profoundly.